Method of testing wafer

ABSTRACT

A method includes following steps. An image of a wafer is captured. A first contact region in the captured image at which the first conductive contact is rendered is identified. A second contact region in the captured image at which the second conductive contact is rendered is identified. The second conductive contact is determined as not shorted to the first conductive contact, in response to the identified second contact region in the captured image is darker than the identified first contact region in the captured image.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a Divisional application of U.S. patent application Ser. No. 17/078,523, filed Oct. 23, 2020, which is a Continuation application of U.S. patent application Ser. No. 15/486,598, filed Apr. 13, 2017 (now U.S. Pat. No. 10,818,595, issued Oct. 27, 2020), which claims priority to U.S. Provisional Application Ser. No. 62/427,277, filed Nov. 29, 2016, all of which are herein incorporated by reference in their entireties.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapid growth. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of processing and manufacturing ICs.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a plan view of dummy structure layouts in accordance with some embodiments of the present disclosure.

FIG. 2 is a perspective view of a dummy structure in accordance with some embodiments of the present disclosure.

FIG. 3 is a cross-sectional view taken along line 3 in FIG. 2.

FIG. 4 is a cross-sectional view of a dummy structure with an unwanted electrical connection between contacts in accordance with some embodiments of the present disclosure.

FIG. 5 to FIG. 8 illustrate a method for manufacturing a dummy structure at various stages in accordance with some embodiments of the present disclosure.

FIG. 9 is a cross-sectional view of a dummy structure in accordance with some embodiments of the present disclosure.

FIG. 10 is a cross-sectional view of a dummy structure with an unwanted electrical connection between contacts in accordance with some embodiments of the present disclosure.

FIG. 11 is a cross-sectional view of a dummy structure in accordance with some embodiments of the present disclosure.

FIG. 12 is a cross-sectional view of a dummy structure with an unwanted electrical connection between contacts in accordance with some embodiments of the present disclosure.

FIG. 13 is a cross-sectional view of a dummy structure in accordance with some embodiments of the present disclosure.

FIG. 14 is a cross-sectional view of a dummy structure with an unwanted electrical connection between contacts in accordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

FIG. 1 illustrates a plan view of dummy structure layouts in accordance with some embodiments of the present disclosure. With reference to FIG. 1, there is shown some embodiments comprising a plurality of individual dies 101 on a wafer, wherein each die 101 includes a substrate (not shown) having electronic devices formed thereon. The substrate is covered with one or more dielectric layers and one or more metal layers (not shown) between adjacent dielectric layers. The metal layers provide connectivity and routing for the underlying electronic devices. There can be any number of alternating layers of metal layers and dielectric layers located on the die 101, but a range of layers would be from three layers to twelve layers of alternating metal layers and dielectric layers.

Separating the individual dies 101 are scribe lines 103, which form an area for a saw to be used to separate the die 101 from the wafer. The scribe lines 103 can be referred to as a scribe region. In some embodiments, scribe lines 103 run substantially parallel to the edges of the individual dies 101. Accordingly, the scribe lines 103 intersect each other at the corners of the dies 101 to form a junction region.

The scribe lines 103 are formed by not placing functional structures (structures that will be used by the die 101 once it has been cut from the wafer) into the area intended for the scribe line 103. Other structures, such as test pads or dummy metals used for planarization, could be placed into the scribe line 103, but would not be necessary for the functioning of the die 101 once the die 101 has been cut from the wafer. The scribe lines 103 may have a width of between about 20 μm and about 180 μm. Within the scribe lines 103 is placed at least one dummy structure 100.

FIG. 2 is a perspective view of a dummy structure 100 in accordance with some embodiments of the present disclosure. As shown in FIG. 2, the dummy structure 100 includes a substrate 110 and an isolation structure 120 in a trench of the substrate 110. The substrate 110 may comprise a semiconductor material such as silicon, germanium, diamond, or the like. Alternatively, compound materials such as silicon germanium, silicon carbide, gallium arsenic, indium arsenide, indium phosphide, silicon germanium carbide, gallium arsenic phosphide, gallium indium phosphide, combinations of these, and the like, may also be used. Additionally, the substrate 110 may comprise a silicon-on-insulator (SOI) substrate. Generally, an SOI substrate comprises a layer of a semiconductor material such as epitaxial silicon, germanium, silicon germanium, SOI, silicon germanium on insulator (SGOI), or combinations thereof. The substrate 110 may be doped with a p-type dopant or an n-type dopant.

The isolation structure 120 in the trench of the substrate 110 may be a shallow trench isolation (STI) structure embedded in the substrate 110, and is referred to as an STI structure 120 hereinafter. The STI structure 120 includes a material such as silicon oxide, silicon nitride, silicon oxynitride, low-k materials, other suitable materials, or any combinations thereof. In some embodiments, an optional thermal oxide trench liner is grown to improve the trench interface.

The dummy structure 100 includes first contacts 142 and 144 on a first active region (OD) 112 a of the substrate 110, second contacts 132 and 134 on the STI structure 120, and third contacts 152 and 154 on a second active region 112 b of the substrate 110. For example, the first contacts 142 and 144 are in contact with the first active region 112 a, the second contacts 132 and 134 are in contact with the STI structure 120, and the third contacts 152 and 154 are in contact with the second active region 112 b, and hence the first and third contacts 142, 144, 152, and 154 have electrical resistances different from electrical resistances of the second contacts 132 and 134. More particularly, the first contacts 142 and 144 are electrically isolated from the second contacts 132 and 134 at least by the STI structure 120, and the third contacts 152 and 154 are electrically isolated from the second contacts 132 and 134 at least by the STI structure 120 as well. Such an electrical isolation between the first and second contacts 142, 144, 132, 134 and between the third and second contacts 152, 154, 132, and 134 may be advantageous to detect an unwanted electrical connection between the first and second contacts or between the third and second contacts.

For example, scanning electron microscope (SEM) can be used to detect the unwanted electrical connection between contacts. As illustrated in FIG. 3, which is a cross-sectional view taken along line 3 in FIG. 2, an electron beam can be applied to the first and third contacts 140 and 150 (i.e. the first contacts 142 and 144 and the third contacts 152 and 154 in FIG. 2) landing on the first and second active regions 112 a and 112 b, and applied to the second contacts 130 (i.e. the second contacts 132 and 134 in FIG. 2) landing on the STI structure 120. In the depicted embodiments where the first, second, the second contacts 130 are electrically isolated from the first and third contacts 150. Therefore, electrons cannot be discharged from the substrate 110 due to electrical isolation of the STI structure 120. The electron beam does not penetrate the second contacts 130 when electron charge is accumulated therein, such that the number of second electrons detected from the second contacts 130 is decreased. In contrast, connection between the first contacts 140 and first active region 112 a releases electrons through the substrate 110, and connection between the third contacts 150 and second active region 112 b releases electrons through the substrate 110. Therefore, images of the second contacts 130 observed using SEM are darker than images of the first and third contacts 140 and 150 observed using SEM.

On the other hand, as shown in FIG. 4, which illustrates an unwanted electrical connection P connected between the first and second contacts 140 and 130, the first and second contacts 140 and 130 are not electrically isolated due to the unwanted electrical connection P. Therefore, when an electron beam is applied to the first and second contacts 140 and 130, no charge or less charge accumulates on the second contact 130 because the unwanted electrical connection P allows electrical connection between the first and second contacts 140 and 130. Electrons from the second contact 130 thus transfer to the unwanted electrical connection P, and discharge together with the electrons from the first contact 140 through the substrate 110. Therefore, an image of the second contact 130 observed using SEM may have substantially the same brightness as images of the first and third contacts 140 and 150 observed using SEM.

As stated above, an SEM image of the second contact 130 has substantially the same brightness as SEM images of the first and third contacts 140 and 150 when the unwanted electrical connection P exists, but an SEM image of the second contact 130 is darker than SEM images of the first and third contacts 140 and 150 when the unwanted electrical connection P does not exists. Accordingly, the unwanted electrical connection P can be detected or inspected according to brightness contrast between the first and second contacts 140 and 130 or between the second and third contacts 130 and 150. In some embodiments, detection of the unwanted electrical connection P is not limited to usage of the scanning electron microscope, other devices having an electron beam can also be utilized as an inspection device.

In some embodiments, the unwanted electrical connection P may be a piping defect in an interlayer dielectric (ILD) layer 170 between the first and second contacts 140 and 130. More particularly, with reduced process size and increased integration, voids with piping shapes (not shown) easily form among gates due to the low filling capability of the ILD layer 170, and the voids may connect contact holes. As the contact holes are filled with a conductive material to form contacts, the conductive material fills the voids as well, and thus the conductive material in the voids forms the unwanted electrical connection P, which can be referred to as piping defects. Some contacts (e.g. first and second contacts 140 and 130 in FIG. 4) thus connect to each other due to the piping defects. When an electron beam is applied to the first, second, and third contacts 140, 130, and 150, images of the first and second contacts 140 and 130 may have substantially the same brightness if the piping defect exists therebetween, and the piping defect can thus be detected.

In some embodiments, as shown in FIG. 2, gate structures 190 are over the first and second active region 112 a and 112 b. At least one gate structure 190 is between the first contacts 142 and 144, and hence portions of the first active region 112 a under the first contacts 142 and 144 can be doped as source/drain regions using an n type dopant or a p type dopant. Similarly, at least one gate structure 190 is between the third contacts 152 and 154, and hence portions of the second active region 112 b under the third contacts 152 and 154 can be doped as source/drain regions using an n type dopant or a p type dopant. The first and third contacts 142, 144, 152, and 154 landing on the source/drain regions are located on the scribe lines 103 (shown in FIG. 1), and hence the source/drain regions and the gate structure 190 therebetween may not act as functional transistors. In some embodiments, the gate structure 190 extends over the STI structure 120 and is also between the second contacts 132 and 134. As a result, first, second and third contacts are present between the gate structures 190 so as to detect the piping defect between the gate structures 190. In some embodiments, the gate structure 190 may include a stack of a gate dielectric layer 192 and a gate electrode 194, as illustrated in FIG. 2. Some elements, such as gate spacers on opposite sides of the gate structure 190 and ILD layers over the substrate 110, are not illustrated in FIG. 2 for clarity.

The first, second and third contacts can be formed using, for example, a damascene process as shown in FIG. 5 to FIG. 8. As shown in FIG. 5, the STI structure 120 is formed in the substrate 110, and the first and second active regions 112 a and 112 b are defined by the STI structure 120. In other words, the STI structure 120 abuts the first and second active regions 112 a and 112 b, and is disposed between the first and second active regions 112 a and 112 b. Formation of the STI structure 120 may include etching the substrate 110 to form a trench 110 t, and filling the trench 110 t with a dielectric material to form the STI structure 120. The STI structure 120 may be formed of silicon oxide deposited by a high density plasma, for example, although other dielectric materials formed according to various techniques may also be used. In some embodiments, top surfaces of the first and second active regions 112 a and 112 b and a top surface of the STI structure 120 may be substantially level with each other, such as by performing a chemical mechanical polish (CMP) process after depositing the material of the STI structure 120, although the surfaces may be at slightly different levels.

Thereafter, one or more gate structures (not shown in this figure) may be formed over the first and second active regions 112 a and 112 b, and source/drain regions can be formed in the first and second active regions 112 a and 112 b before or after the formation of the gate structures, for example, using a suitable technique, such as implant processes or combinations of epitaxy processes and implant processes.

Next, as shown in FIG. 6, the ILD layer 170 is deposited on the substrate 110. Portions of the ILD layer 170 are then etched away where contact holes are to be formed. Continuing in FIG. 7, the ILD layer 170 is subjected to a photolithography and etching process to form contact holes V1, V2 and V3 for the subsequently formed contacts. In some embodiments, the etching of the contact holes V1 and V3 stops on the first and second active regions 112 a and 112 b, and the etching of the contact hole V2 stops on the STI structure 120, and hence the first and second active regions 112 a and 112 b and the STI structure 120 are respectively exposed at bottoms of the via openings V1, V3, and V2.

As illustrated in FIG. 8, the contact holes V1, V2, and V3 are then filled with conductor utilizing suitable deposition processes, such as chemical vapor deposition (CVD), plasma-assisted CVD, sputtering, electroplating, or the like. In some embodiments, the contact holes V1, V2, and V3 are filled with conductor to form a damascene structure over the substrate 110. The amascene structure is then planarized to remove excess conductor from its top surface and to provide a substantially flat surface for the subsequent processing steps. Since the first and second active regions 112 a and 112 b and the STI structure 120 are respectively exposed at bottoms of the contact holes V1, V3, and V2, the first, third, and second contacts 140, 150, and 130 can be in contact with the first and second active regions 112 a and 112 b and the STI structure 120, respectively.

FIG. 9 is a cross-sectional view of a dummy structure 200 in accordance with some embodiments of the present disclosure. As shown in FIG. 9, the dummy structure 200 includes a substrate 210, a first isolation structure 220 in a trench of the substrate 210, and a second isolation structure 222 in another trench of the substrate 210. The first isolation structure 220 is between a first active region 212 a and a second active region 212 b of the substrate 210, and the second isolation structure 222 is between the second active region 212 b and a third active region 212 c of the substrate 210. In other words, the first and second active regions 212 a and 212 b are separated by the STI structure 220, and the second and third active regions 212 b and 212 c are separated by the STI structure 222. The substrate 210 may comprise a semiconductor material as discussed previously. The first and second isolation structures 220 and 222 may be referred to as first and second STI structures 220 and 222, and may comprise dielectric materials as discussed previously. The dummy structure 200 further includes a dielectric structure 290 formed over the second active region 212 b of the substrate 210. The dielectric structure 290 may be a resist protective oxide (RPO), and is referred to as an RPO structure 290 hereinafter. The RPO structure 290 is deposited over substrate 210 and then selectively removed where silicidation is desired. The RPO structure 290 will prevent silicidation where it remains over the substrate 210.

The dummy structure 200 includes first and third contacts 240 and 250 on the first and third active region 212 a and 212 c of the substrate 210, and second contacts 230 on the RPO structure 290. For example, the first and third contacts 240 and 250 are in contact with the first and third active regions 212 a and 212 c respectively, and the second contacts 230 are in contact with the RPO structure 290, and hence the first and third contacts 240 and 250 may have electrical resistances different from electrical resistances of the second contacts 230. More particularly, the first and third contacts 240 and 250 are electrically isolated from the second contacts 230 at least by the RPO structure 290. Such an electrical isolation between the first and second contacts 240 and 230 and between the third and second contacts 250 and 230 may be advantageous to detect an unwanted electrical connection between the first and second contacts 240 and 230 or between the third and second contacts 250 and 230.

For example, in the depicted embodiments where the first and second contacts 240 and 230 are free from unwanted electrical connections (e.g. piping defects), the first and second contacts 240 and 230 can be electrically isolated at least by the RPO structure 290. Therefore, when the first and second contacts 240 and 230 are observed by SEM or other suitable techniques, the image of the second contact 230 may be darker than the image of the first contact 240. On the other hand, as shown in FIG. 10, which illustrates an unwanted electrical connection P (e.g. a piping defect) connected between the first and second contacts 240 and 230, the first and second contacts 240 and 230 are not electrically isolated due to the unwanted electrical connection P. Therefore, when the first and second contacts 240 and 230 are observed by SEM or other suitable techniques, the images of the first and second contacts 240 and 230 may have substantially the same brightness. As a result, the unwanted electrical connection P can be detected according to brightness contrast between images of the contacts. Formation of ILD layer 270, the first, second and third contacts 240, 230, and 250 may comprise a damascene process, for example, as shown in FIGS. 5-8.

FIG. 11 is a cross-sectional view of a dummy structure 300 in accordance with some embodiments of the present disclosure. As shown in FIG. 11, the dummy structure 300 includes a substrate 310, a first isolation structure 320 in a trench of the substrate 310, and a second isolation structure 322 in another trench of the substrate 310. The substrate 310 may comprise a semiconductor material as discussed previously. The first and second isolation structures 320 and 322 may be referred to as first and second STI structures 320 and 322, and may comprise a dielectric material as discussed previously. The dummy structure 300 further includes first, second, and third doped active regions 312 a, 312 b, and 312 c formed in the substrate 310 using implant processes. Any neighboring two of the first, second, and third doped regions 312 a, 312 b, and 312 c are doped with different types of impurities (or dopants). For example, the first and third doped regions 312 a and 312 c can be doped with n-type impurities, such as phosphorus or arsenic, such as boron or BF₂, and the second doped region 312 b can be doped with a p-type impurity, and vice versa.

The dummy structure 300 includes first contacts 340 on the first doped region 312 a, second contacts 330 on the second doped region 312 b, and third contacts 350 on the third doped region 312 c. For example, the first contacts 340 are in contact with the first doped region 312 a, and the second contacts 330 are in contact with the second doped region 312 b, and the third contacts 350 are in contact with the third doped region 312 c, and hence the first and third contacts 340 and 350 may have electrical resistances different from electrical resistances of the second contacts 330. Such an electrical resistance difference between the first and second contacts 340 and 330 may be advantageous to detect an unwanted electrical connection between the first and second contacts 340 and 330. Similarly, the electrical resistance difference between the second and third contacts 330 and 350 may be advantageous to detect an unwanted electrical connection between the second and third contacts 330 and 350.

For example, in the depicted embodiments where the first and second contacts 340 and 330 are free from unwanted electrical connections (e.g. piping defects), when the first and second contacts 340 and 330 are observed by SEM or other suitable techniques, the images of the first and second contacts 240 and 230 may have different brightness due to the electrical resistance difference between the first and second contacts 240 and 230. On the other hand, as shown in FIG. 12, which illustrates an unwanted electrical connection P (e.g. a piping defect) connected between the first and second contacts 340 and 330, the first and second contacts 340 and 330 are not electrically isolated due to the unwanted electrical connection P. Therefore, when the first and second contacts 340 and 330 are observed by SEM or other suitable techniques, the images of the first and second contacts 340 and 330 may have substantially the same brightness. As a result, the unwanted electrical connection P can be detected according to brightness contrast between images of the contacts. Formation of ILD layer 370, the first, second and third contacts 340, 330, and 350 may comprise a damascene process, for example, as shown in FIGS. 5-8.

FIG. 13 is a cross-sectional view of a dummy structure 400 in accordance with some embodiments of the present disclosure. As shown in FIG. 13, the dummy structure 400 includes a substrate 410 and an isolation structure 420 in a trench of the substrate 410. The substrate 410 may comprise a semiconductor material as discussed previously. The isolation structure 420 may be referred to as an STI structure 420 and may comprise a dielectric material as discussed previously. The STI structure 420 has a top surface in a position lower than top surfaces of first and second active regions 412 and 414 of the substrate 410, and hence the first and second active regions 412 and 414 can be formed as fin-shaped structures protruding from the top surface of the STI structure 420, and hence the first and second active regions 412 and 414 can be referred to as first and second semiconductor fins 412 and 414 hereinafter respectively.

As shown in FIG. 13, the dummy structure 400 includes first contacts 440 on the first semiconductor fin 412, second contacts 430 on the STI structure 420, and third contacts 450 on the second semiconductor fin 414. For example, the first contacts 440 are in contact with the first semiconductor fin 412, and the second contacts 430 are in contact with the STI structure 420, and the second contacts 450 are in contact with the second semiconductor fin 414, and hence the first and third contacts 440 and 450 may have electrical resistances different from electrical resistances of the second contacts 430. More particularly, the first and third contacts 440 and 450 are electrically isolated from the second contacts 430 at least by the STI structure 420. Such an electrical isolation between the first and second contacts 440 and 430 and between the third and second contacts 450 and 430 may be advantageous to detect an unwanted electrical connection between the first and second contacts 440 and 430 or between the third and second contacts 450 and 430.

For example, in the depicted embodiments where the first and second contacts 440 and 430 are free from unwanted electrical connections (e.g. piping defects), the first and second contacts 440 and 430 can be electrically isolated at least by the STI structure 420. Therefore, when the first and second contacts 440 and 430 are observed by SEM or other suitable techniques, the image of the second contact 430 may be darker than the image of the first contact 440. On the other hand, as shown in FIG. 14, which illustrates an unwanted electrical connection P (e.g. a piping defect) connected between the first and second contacts 440 and 430, the first and second contacts 440 and 430 are not electrically isolated due to the unwanted electrical connection P. Therefore, when the first and second contacts 440 and 430 are observed by SEM or other suitable techniques, the images of the first and second contacts 440 and 430 may have substantially the same brightness. As a result, the unwanted electrical connection P can be detected according to brightness contrast between images of the contacts. Formation of ILD layer 470, the first, second and third contacts 440, 430, and 450 may comprise a damascene process, for example, as shown in FIGS. 5-8.

Embodiments of the present disclosure allow contacts of the dummy structure to land on features having different electrical resistances. When an electron beam is applied to the contacts landing on features having different electrical resistances, brightness difference between images of the contacts will be observed if there is no unwanted electrical connection (e.g. piping defect) between the contacts. On the other hand, if there is an unwanted electrical connection (e.g. piping defect) between the contacts, substantially the same brightness of images of the contacts will be observed. Therefore, existence of the unwanted electrical connection between contacts can be determined according to brightness contrast between images of the contacts.

According to some embodiments, a method includes capturing an image of a wafer, the wafer comprising a first conductive contact over an active region of the wafer and a second conductive contact over a shallow trench isolation (STI) region abutting the active region; identifying a brightness of a first contact region in the captured image at which the first conductive contact is rendered; identifying a brightness of a second contact region in the captured image at which the second conductive contact is rendered; and in response to the identified brightness of the first contact region in the captured image being substantially the same as the identified brightness of the second contact region in the captured image, determining that the second conductive contact is shorted to the first conductive contact.

According to some embodiments, a method includes capturing an image of a wafer, the wafer comprising a first conductive contact over a first active region of the wafer and a second conductive contact over an oxide formed on a second active region of the wafer; identifying a first contact region in the captured image at which the first conductive contact is rendered; identifying a second contact region in the captured image at which the second conductive contact is rendered; and in response to the identified second contact region in the captured image being darker than the identified first contact region in the captured image, determining that the second conductive contact is not shorted to the first conductive contact.

According to some embodiments, a method includes capturing an image of a wafer, the wafer comprising a first conductive contact over a first doped semiconductor region and a second conductive contact over a second doped semiconductor region, wherein the first doped semiconductor region is of a first conductivity type, and the second doped semiconductor region is of a second conductivity type opposite the first conductivity type; identifying a first contact region in the captured image at which the first conductive contact is rendered; identifying a second contact region in the captured image at which the second conductive contact is rendered; and in response to the identified second contact region in the captured image having a brightness different than a brightness of the identified first contact region in the captured image, determining that the second conductive contact is not shorted to the first conductive contact.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A method, comprising: capturing an image of a wafer, the wafer comprising a first conductive contact over a first active region of the wafer and a second conductive contact over an oxide formed on a second active region of the wafer; identifying a first contact region in the captured image at which the first conductive contact is rendered; identifying a second contact region in the captured image at which the second conductive contact is rendered; and in response to the identified second contact region in the captured image being darker than the identified first contact region in the captured image, determining that the second conductive contact is not shorted to the first conductive contact.
 2. The method of claim 1, further comprising: performing silicidation on the wafer after the oxide is formed on the second active region.
 3. The method of claim 1, wherein the oxide has a top surface higher than the first active region.
 4. The method of claim 1, wherein the second conductive contact over the oxide has a shorter height than the first conductive contact over the first active region.
 5. The method of claim 1, further comprising: in response to the identified second contact region in the captured image having a brightness comparable to a brightness of the identified first contact region in the captured image, determining that the second conductive contact is shorted to the first conductive contact.
 6. A method, comprising: capturing an image of a wafer, the wafer comprising a first conductive contact over a first doped semiconductor region and a second conductive contact over a second doped semiconductor region, wherein the first doped semiconductor region is of a first conductivity type, and the second doped semiconductor region is of a second conductivity type opposite the first conductivity type; identifying a first contact region in the captured image at which the first conductive contact is rendered; identifying a second contact region in the captured image at which the second conductive contact is rendered; and in response to the identified second contact region in the captured image having a brightness different than a brightness of the identified first contact region in the captured image, determining that the second conductive contact is not shorted to the first conductive contact.
 7. The method of claim 6, further comprising: in response to the brightness of the identified second contact region in the captured image being comparable to the brightness of the identified first contact region in the captured image, determining that the second conductive contact is shorted to the first conductive contact.
 8. The method of claim 6, wherein the wafer further comprises a third conductive contact over a third doped semiconductor region of the first conductivity type, the second doped semiconductor region is laterally between the first and third doped semiconductor regions, and the method further comprises: identifying a third contact region in the captured image at which the third conductive contact is rendered; and in response to the brightness of the identified second contact region in the captured image being different than a brightness of the identified third contact region in the captured image, determining that the second conductive contact is not shorted to the third conductive contact.
 9. The method of claim 8, further comprising: in response to the brightness of the identified second contact region in the captured image being comparable to the brightness of the identified third contact region, determining that the second conductive contact is shorted to the third conductive contact.
 10. The method of claim 6, wherein the wafer further comprises a shallow trench isolation (STI) region spacing the first doped semiconductor region apart from the second doped semiconductor region.
 11. A method comprising: forming a first shallow trench isolation (STI) region between a first active region and a second active region of a substrate, and a second STI region between the second active region and a third active region of the substrate; forming a dielectric layer over the second active region; forming a first contact on the first active region and a second contact on the dielectric layer; obtaining an electron microscope image of the substrate; measuring a brightness difference between the first contact and the second contact rendered in the electron microscope image; and determining, based on the measured brightness difference, whether a short circuit occurs between the first contact and the second contact.
 12. The method of claim 11, wherein determining whether a short circuit occurs between the first contact and the second contact comprises: determining the first contact is shorted to the second contact when the measured brightness difference between the first contact and the second contact rendered in the electron microscope image is zero.
 13. The method of claim 11, wherein determining whether a short circuit occurs between the first contact and the second contact comprises: determining the first contact is not shorted to the second contact when the measured brightness difference between the first contact and the second contact rendered in the electron microscope image is non-zero.
 14. The method of claim 11, wherein determining whether a short circuit occurs between the first contact and the second contact comprises: determining the first contact is not shorted to the second contact when the second contact rendered in the electron microscopic image is darker than the first contact rendered in the electron microscopic image.
 15. The method of claim 11, further comprising: prior to obtaining the electron microscope image of the wafer, forming an interlayer dielectric (ILD) layer over the substrate; etching the ILD layer to form a first contact hole and a second contact hole extending through the ILD layer, wherein the first active region is exposed at a bottom of the first contact hole, and the dielectric layer is exposed at a bottom of the second contact hole; and forming the first and second contacts in the first and second contact holes, respectively.
 16. The method of claim 15, wherein forming the first and second contacts comprises: depositing a conductor to fill the first and second contact holes, such that the conductor is in contact with the first active region at the bottom of the first contact hole and also in contact with the dielectric layer at the bottom of the second contact hole; and planarizing the conductor to expose the ILD layer.
 17. The method of claim 11, further comprising: forming a third contact on the third active region.
 18. The method of claim 11, further comprising: performing a silicidation process on the substrate after the dielectric layer is formed on the second active region.
 19. The method of claim 11, wherein the second contact has a bottom surface higher than a bottom surface of the first contact.
 20. The method of claim 11, wherein the second contact has a shorter height than the first contact. 